In shaft furnaces, reducing gas is generally injected into the shaft furnace for aiding the reduction of ore in the shaft furnace.
The injected reducing gas may be formed by mixing two separate gas flows before injection. This may be done in order to obtain a desired gas composition or gas temperature. The mixing necessitates controlling the feeding of two separate gas flows to a mixing chamber. Generally, control valves such as e.g. butterfly valves, are arranged in the feeding conduits of the gas flows, so as to allow the correct amount of gas from each gas flow into the mixing chamber and thereby obtain the desired mixing rate of the two separate gas flows.
In applications where the incoming gas flow comprises an aggressive gas or a particularly hot gas, the control valve is exposed to such extreme conditions that the correct working and the lifetime of the control valve is compromised. The incoming gas flow may e.g. comprise a recycled furnace top gas at a temperature above 1000° C.
Control valves exposed to high temperatures are usually provided with a cooling system in order to prevent damage to the control valve through the high gas temperature. An undesired effect of this is that the gas temperature may be lowered as it passes through the control valve. Thermal insulation, which may be provided on the control valve, comprises a variety of different materials and has to be heat resistant and at the same time allow fast gas temperature and pressure changes. Last but not least, the control valve should have good sealing properties when closing and ensure low pressure losses while regulating the gas flow rate.
The reliability and durability of control valves is compromised by the exposure to such extreme conditions. Such control valves not only have high manufacturing costs, but also require intensive and frequent maintenance operations.